Glycoalkaloids are members of plant-derived compounds and are said to be steroidal alkaloids. It has been reported that the glycoalkaloid structure has an N atom and is an isoprenoid having 27 carbon chains, and there are 422 compounds of glycoalkaloids from plants belonging to the genus Solanum (Non-Patent Literature 1: Chapter 7.8). In addition to solanaceous plants belonging to the genus Solanum, plants belonging to the family Liliaceae are known to contain glycoalkaloids. Important glycoalkaloids are chaconine and solanine from potatoes (Solanum tuberosum) belonging to the genus Solanum of the family Solanaceae and tomatine from tomatoes (Solanum lycopersicum).
The potato is the world's forth largest crop produced, following maize, rice, and wheat. It is well known that buds sprouting from the tubers and aerial parts of the potato contain chaconine and solanine, which are toxic substances. Chaconine and solanine cause toxic symptoms such as abdominal pain, dizziness, and mild disturbances of consciousness. Also, chaconine and solanine are likely to accumulate in tubers as a result of damage or exposure to sunlight. Therefore, there is the risk of accidental poisoning as a result of a failure in tuber management. Accidental glycoalkaloid poisoning sometimes occurs. In a recent case, accidental glycoalkaloid poisoning occurred in an elementary school in Nara city in Japan on Jul. 16, 2009 (reported by Asahi.com). The glycoalkaloid levels in potato tubers are controlled at 20 mg/100 g or lower by, for example, storing the tubers in dark places. Thus, in general, potato tubers are safe food products. However, in consideration of the risk of accidental poisoning described above, reduction of potato glycoalkaloid content is a key issue for those involved in breeding, production, storage, transportation, distribution, or purchasing in the potato-related industry. However, reduction of potato glycoalkaloid content has not been achieved thus far. This is because there is no wild-type potato line free from glycoalkaloids, the glycoalkaloid biosynthesis pathway has not been elucidated (Non-Patent Literature 1 (FIGS. 7.24 A and B) and Non-Patent Literature 2), and there has been little progress in identification of genes involved in the biosynthesis pathway.
It is known that glycoalkaloids have medicinal properties such as anticancer activity, liver-protecting effects, antispasmodic effects, immune-system-promoting effects, antifungal effects, antiprotozoal effects, and molluscicide activity, in addition to poisonous properties such as anticholinesterase activity and membrane disruption effects (Non-Patent Literature 1). It has been reported that esculeoside A, which is a glycoalkaloid metabolite, shows anti-arteriosclerotic effects in tomatoes (Non-Patent Literature 3). However, since the biosynthesis pathway has not been elucidated, there has been substantially no advance in research and development to suppress or efficiently produce metabolites.
In recent years, there have been some reports on genes involved in the glycosylation after transfer of a sugar to aglycone (Non-Patent Literature 4-6). Non-Patent Literature 4 reports that a UDP-galactosyltransferase gene is involved in the pathway of formation of γ-solanine from solanidine (aglycone), and it also reports a strain in which the gene is suppressed. However, suppression of chaconine has been never achieved (Non-Patent Literature 4 (FIG. 2)). Non-Patent Literature 4 reports that a UDP-glucosyltransferase gene is involved in the pathway of formation of γ-chaconine from solanidine, and it also reports a strain in which the gene is suppressed. However, suppression of chaconine and solanine has been substantially impossible (Non-Patent Literature 5 (FIG. 5)). Non-Patent Literature 6 reports a rhamnosyltransferase gene involved in the pathway of formation of α-chaconine from β-chaconine and the pathway of formation of α-solanine from β-solanine. In this case, β-isomer or γ-isomer increases while α-isomer decreases. Thus, it is understood that it has been very difficult to control total glycoalkaloid content, even though it has become possible to change the molecular species of glycoalkaloid by suppressing the glycosylation.
There is a report of an attempt to reduce glycoalkaloid through overexpression of genes involved in biosynthesis of plant sterols and plant hormones (Non-Patent Literature 7). However, in such case, it was merely possible to reduce the glycoalkaloid content up to almost half the initial amount (Non-Patent Literature 7 (FIG. 5)).